Gain control circuits



U 27, 1940- H. L. BARNEY ET A1. 213,034

GAIN CONTROL CIRCUITS f Filed July 15,' 1959 AM. C UR 77S BV condensers.

Patented Aug. 27, 1940 UNITED STATES 2,213,034 APATENT OFFICE GAINCONTROL CIRCUITS Harold L. Barney and Bjorn G. Bjornson, New

York, N. Y., and Austen M. Curtis, South 0range, N. J., assignors toBell Telephone Laboratories, Incorporated, New York, N. Y., acorporation of New York Application July 15, 1939, Serial No. 284,584

9 Claims.

-gain control circuitsfor a receiving station on a signal transmissionline that shall have a forward-acting gain increaser circuit undercontrol of syllabic impulses to raise the gain of the signals receivedover the transmission line only on the rising parts of the syllabicimpulses.

A further object of the invention is to provide gain control circuitsfor maintaining substantially constant volume at a receiving station ona signal transmission line that shall have a backward-acting gaindecreaser circuit for controlling a vario-repeater on the line to lowerthe gain in case the, signals go, above a predetermined level and thatshall have a forward-acting gain increaser circuit under control ofsyllabic impulses to raise the gain of the signals received over thetransmission line only on the rising part of the syllabic impulses.

On long signal circuits 'and particularly on circuits having a longradio link, trouble is .often experienced at a receiving station byreason of the so-called` fading produced during transmission.

It is desirable in signal circuits of the above-indicated character tocorrect for the fading and to hold the received signals at'substantiallyconstant volume. l,

In gain control circuits constructed in accordance with the invention,gain increase in the case of low received signa-ls is effected at arelatively quick rate under control of syllabic frequency impulses. 'Thegain controlling operation is only effected under control of the risingportion of ka syllabic impulse. Furthermore, the gain increase iseffected by the gain increaser circuit at dilferent rates, dependingonthe input signal amplitude.

At the receiving station on a long transmission path avario-repeater isprovided for varying the gain. The vario-repeater may be in any suitableform and preferably comprises a pair of pushpull tubes of any suitabletype and a varistor. The varistor preferably comprises a bridge ofcopper-oxide elements shunted across the transmission path. The controlgrids of the two tubes connected in push-pull arrangement arecontrolled` according to the charge on two control One of the condensersis relatively small compared to the othercondenser. Two condensers ofthis typefor controlling a variorepeater are disclosed in the patent toS. Doha, Jr., No. 2,156,622, May 2, 1939. The varistor in the form of acopper-oxide bridge is controlled according to the space current flowingthrough the tubes in push-pull relationship. When the tubes have largeimpedance andthe space current iiow through them is small, the shuntingaction effected by the copper-oxide bridge is large.-

A backward-acting gain decreaser circuit is provided for impressing anegative charge on the control condensers to lower the gain on thetransmission line. A forward-acting gain increaser circuit is providedfor impressing a positivev charge on the control condensers to raise thegain on the transmission line. The gain decreaser circuit and the gainincreaser circuit are controlled according to the energy level of thesignals on the transmission line.

Thegain decreaser circuit which is connected to the transmission linebeyond the vario-repeater comprises a high-pass filter which preventsthe operation of the gain decreaser circuit by low frequency transientsdue to unbalanced plate current inthe vario-repeater. The low frequencytransients might otherwise cause excessive reductions of gain. Thefilter is connected to a three-element cold cathode tube by means of atransformer. tube' when broken down impresses a negative charge on thecontrol condensers to govern the operation of the push-pull tubes in thevariorepeater. The push-pull tubes when operated by the controlcondensers also control the copper-f oXide bridge to lower the gain onthe transmission line.

The gain increaser circuit is connected to the transmission line beforethe vario-repeater and comprises a suitable lter which selects currentshaving a range of frequencies usually in the center portion of the voicefrequency range. In systems of transmission where the voice currents ofdifferent frequencies are not displaced, the selected frequencies arebetween 800 and 1600 cycles per second. However, in systems Where thevoice frequencies have been displaced from their normal positions as iscustomary in systems of primary transmission, the pass-band of thefilter is correspondingly displaced. The filter is connected to anamplifier having a limiter of the peak chopper type in the outputcircuit thereof. The output circuit of the' amplifier is connected tothe diode portion of a double diode-triode.

The input signals, when rectified by the diodef s portion of the doublediode-triode, supply a negative bias for controlling the grid of thetriode. A filter in the output circuit of the triode portion of thediode-triode selects a range of syllabic frequency which is preferablycentered around The three-element cold cathode Iii) six or eight cyclesper second. The syllabic frequency lter is connected by a transformer toa coil of a gain increaser relay. A rectifier element which ispreferably of the copper-oxide type is connected in series with the coilof the gain increaser relay in order to effect operation of the relayonly on the rising part of a syllabic impulse. A second copper-oxideelement is connected in shunt across the circuit of the coil for thegain increase relay to insure that the gain increaser relay is notoperated on the falling portion of a syllabic impulse.

The gain increaser relay when operated impresses a positive charge onthe control condensers to raise the gain on the transmission line. Again rate increase relay which is operated in accordance with the energyvalue of the syllabic frequency impulses controlling the gain increaserrelay is provided for reducing the rate of gain increase effected by thegain increaser circuit whenever the signals on the transmission linebefore the vario-repeater are above a predetermined level. The gainincrease is controlled by syllabic impulses on a gain increaser circuitin order to insure against false operation of the gain increaser circuitby noise or static disturbances. 'Ihe gain increase is controlled onlyon a rising portion of a syllabic impulse, so that in so far aspossible, the gain increase action may be performed during the initialpart of the speech syllable. Thus, when the gain is sufficiently high sothat the rest of the speech syllable operates the gain decreaser, nofurther gain increase is likely to take place on the final part of thesyllable.

'I'he single figure ln the accompanying drawing is a diagrammatic viewof gain control circuits constructed in accordance with the invention.

Referring to the drawing, a gain control arrangement I is shown havinginput conductors 2 and 3 and output conductors 4 and 5. The inputconductors 2 and 3 are connected to a source of signals varying inintensity and which may be the output of a radio receiver (not shown)The output conductors 4 and 5 may be connected to a telephone line or toa loud-speaker (not shown) to which it is desired to apply signals of arelatively constant intensity. A vario-repeater 6 comprising two tubes 1and 8 and a copper-oxide bridge 9 is provided in the path between theinput conductors 2 and 3 and the output conductors 4 and 5 forcontrolling the gain effected. The tube 1 comprises a controlv grid I0,a cathode II, an anode I2, a screen grid I3 and a suppressor grid I4.The tube 8, which is similar to the tube 1, comprises a control grid I5,a cathode I6, an anode I1, a screen grid I8 and a suppressor grid I9.The copper-oxide varistor bridge 9 is composed of copper-oxide elementsshunted across the transmission path. The input conductors 2 and 3 areconnected to the varistor 9 by means of a transformer 20. Resistanceelements 2l are disposed on each side of the bridge 9. A transformer 22connects the pad formed by the varistor 9 and the resistance elements 2|to the input circuits of the tubes 'I and 8 in push-pull relationship.The output circuits of the tubes 'I and 8 are connected by a transformer23 to a negative feedback amplifier 24. The output circuit of theamplifier 24 is connected by a hybrid" coil 25 to the output conductors4 and 5 and to a gain decreaser circuit 26. f

Two control condensers 21 and 28 are provided for controlling the gridsI0 and- I5 of the tubes 1 and 8. The condenser 21 is relatively smallcompared to the condenser 28 and serves to effect quick changes in gainof short duration on the transmission path, The larger condenser 28serves to control the tubes 1 and 8 in the case of prolonged changes inVolume on the transmission path. Condenser 28 in series with theresistance elements 29, 30 and 3I is connected in shunt to the condenser21 so that the small charges impressed on the condenser 21 may betransferred to the larger condenser 28. A battery 32 is provided forsupplying anode potential to the tubes 1 and 8. Battery 32 also providespotential for governing the operation of the copper-oxide bridge 9. Abattery 33 supplies potential for heating the filaments of the tubes 'Iand 8 and the amplifier 24. The battery 33 also supplies potential forbiasing the bridge 9.

The gain decreaser circuit 26 comprises a filter 34 connected to thehybrid coil 25. The lter 34 is connected by a transformer 35 to athreeele ment cold-cathode tube 36. The tube 36 comprises threeelectrodes 31, 38 and 39. The breakdown circuit for the cold-cathodetube 36 may be traced from one terminal of the secondary winding of thetransformer 35 through the electrode 31, electrode 38 and resistance 40to the other terminal of the secondary winding of the transformer 35.The operating circuit for the cold-cathode tube 36 may be traced fromthe z electrode 39 through resistance 3|, condenser 21, resistance 4I,and secondary winding for the transformer 35 to the electrode 31 of thetube 36. A shunt circuit, comprising resistance element 3I, resistanceelement 30, resistance element 29 and condenser 28, is connected acrossthe condenser 21. Potential drop across the resistance 4I in theoperating circuit of the tube 36 supplies potential for biasing the tube36. Op eration of the control gap immediately starts operation in themain gap for impressing a negative charge on the condenser 21. Thepotential drop across the resistance 4I is provided by a circuitextending from the battery 33 through the resistance 4I, bridge 9 andground return to other terminals of the battery 33.

In case the signals on the transmission line beyond the vario-repeater 6go above a predetermined level, the cold-cathode tube 36 is broken downfor completing the circuit for charging the lcontrol condenser 21 withnegative potential.

The negative potential on the control condenser 21 is impressed on thegrids I0 and I5 of the tubes 1 and 8 to increase the impedance of thetubes and decrease the space current. The bridge 9 is controlled inaccordance with the space current fiow through the tubes 1 and 8. Thecircuit for the space current through tubes 1 and 8 may be traced fromone terminal of the battery 32 through a choke coil 42, the primarywinding of the transformer 23, plates I 2 and I1 and cathodes II and I6of tubes 1 and 8, resistance element 43, resistance element 4I, battery33 and ground return to the battery 32. The potential drop across theresistance 4I formed by the space current ow through the tubes 1 and 8cooperates with the potential drop across resistance 4I in circuit withthe battery 33 to control the copper-oxide bridge 9. The current flowthrough the bridge 9 from the battery 33 lowers the impedance of thebridge across the transmission path. The current flow Afrom the battery32 through the tubes 1 and 8 and resistance element 4I produces apotential drop across the resistance element 4I which reduces the effectof the battery 33 on the bridge 9. Thus, the bridge 9 is controlledaccording to the spacecurrent flow through the tubes 1 and 8. If thespace current ow through the tubes 1 and 8 is reduced by impressingnegative potential on the grids l and I5 as in the case underconsideration, then the resistance of the bridge is reduced to increasethe shunting action thereof.

A gain increaser circuit 44 is provided for impressing a positive chargeon the control condensers 21 and 28 to increase the gain on thetransmission path effected by the vario-repeater 6. The gain increasercircuit 44 comprises a filter 45 connected to the input conductors 2 and3 ahead of a vario-repeater 6. The lter 45 selects a frequency bandwithin the voice frequency range which is centered around a frequency of1200 cycles. The filter 45 is connected by a transformer 46 to the inputcircuit of an amplifier 41 of anysuitable type. The output circuit ofthe amplifier 41 is connected by a transformer 48 to the diode portionof a diode-triode 49. The diodetriode 49 comprises control grid 50,cathode 5|, an anode 52 and auxiliary anodes 53. A resistance 54 and acondenser 55 are shunted across the secondary Winding of transformer 48to properly terminate the Winding and give the transformer the desiredfrequency characteristic.

The rectifier circuit completed by the diode portion of the diode-triode49 may be traced from one terminal of the secondary Winding of thetransformer 48 through the auxiliary anodes 53, cathode 5|, resistanceelement 56 and coupling resistance 51, to the other terminal of thesecondary Winding of the transformer 48. When a` suitable alternatingcurrent is impressed on the gain increaser circuit 44, the resistanceelement 51 shunted by the condenser 58, and the condenser 59 in serieswith the resistance element 60 impresses a steady or slowly varyingnegative potential on the control grid 50 of the triode portion of thediode-triode. The output from the triode portion of the diode-triodetube 49 is supplied to a gain increaser relay 6| and a gain rate relay62. The gain increaser relay 6| comprises a biasing winding 63 and anoperating winding 64. The gain rate relay 62 comprises a biasing windingand an operating Winding 66. The biasing windings 63 and 65 of therelays 6| and 62 are energized from a battery 61.

' A transformer 68 connects the output circuit of the triode portion ofthe diode-triode tube 49 to the operating coil 64 of the gain increaserrelay 6|. A condenser 69 is connected across the primary winding of therelay 68 in order to tune the circuit to syllabic frequencies.Preferably, the circuit is tuned to six or eight cycles. A copper-oxidevaristor element 19 is connected in the circuit between the operatingcoil 64 ol the gain decreaser relay 6| and the secondary Winding of thetransformer 68 to operate the re- 'lay 6| on the rising portion of asyllabic impulse.

AAsecond copper-oxide rectifier element 1| is shunted across thesecondary winding of the transformer 68 in order to insure that therelay 6| cannot be operated on the falling portion of a syllabicimpulse.

The gain increaser relay when operated moves y'an armature 12 fromengagement with a contact member 13 into engagement with a contactmember 14. Upon engagement of the armature 12 with the contact member14, a circuit is completed for impressing a positive charge on thecontrol condensers 21 and 28. Two resistance elements 15 and 16 areconnected in circuit with the battery 61 and the drop across theresistance condenser 28,

elements 15 and 16 is employed forcharging the control condensers 21 and28 with positive potential. The circuit for charging the condensers 21and 28 may be traced from the resistance elements 16 and 15 throughcontact member 11, armature 18 of the relay 62, resistance element 19,armature 12, contact member 14, resistance element 80, resistanceelements 30 and 3| and condenser 21 in shunt with the resistance 29 andresistance 4|, battery 33 and ground return to the resistance elements16 and 15. A battery is connected through a resistance 86 to the contactmember 13 for impressing a negative potential on the armature 12. Theimpressing of a negative charge on the armature 12 prevents the leakageof any positive charge to the contact member 14 from the resistanceelements 15 and 16. A resistance element 81 provides a leak for thecharge on the condensers 21 and 28. trolled by the potential drop acrossthe resistance element 88.

The operating coil 66 o-f the gain rate relay 62 is connected in serieswith the primary winding of the transformer 68 in order to be opreleaseof the relay 62 the armature 18 disengages the Contact member 11 andengages contact member 8|. This operation of the gain rate relay 62serves to change the rate of gain increase effected by the gainvincreaser circuit 44. The condensers 21 and 28 are now charged only kbythe drop across the resistance element 16 in place of the drop acrossresistance elements 15 and 16, as formerly.

When signals are received from the transmission path, the filter 45selects the desired range of voice frequency components, which areamplied by amplifier 41. A peak limiter 83 in the output circuit ofamplifier 41 is provided for chopping off extremely high peaks and forlimiting the overloading of the tube 49. The output f from the amplifier41 is rectified by the diode portion of the diode-triode 49 to supplynegative bias to the control grid of the triode portion of thediode-triode. A frequency of six or eight cycles, due to syllabicmodulation of the Voice y currents is selected in the output circuit ofthe triode portion of the diode-triode 49 for operating the gainincreaser relay 6|. Each time the gain increaser relay 6| is operated,it impresses an increment of positive charge on the control condensers21 and 28. creases the negative charge on the control condensers and onthe grids I0 and I5 of the tubes 1 and to decrease the impedance of thetubes 1 and v8 and increase the space current flow therethrough. Thespace current flow through the tubes 1 and 8 in turn decreases the shuntimpedance of the copper-oxide bridge 9. If the signals on thetransmission line before the variorepeater are above a predeterminedlevel, the gain control relay 62 is operated to decrease the rate atwhich the control condensers 21 and 28 are charged.

Modifications in the circuits and in the arrangement and location ofparts may be made The voltage finallyA arrived at is conn Thisprogressively dewithin the spirit and scope of the invention and suchmodifications are intended to be covered by the appended claims.

vWl'iat is claimed is:

1. In a gain adjusting circuit, a vario-repeater lfor controlling thegain on a signal circuit, means comprising a forward-acting gainincreaser circuit for governing said vario-repeater under 'control ofsyllabic impulse to raise the gain on the signal circuit, and means forinsuring control of the vario-repeater by the gain increaser circuitonly on the rising portion of a syllabic impulse.

2. In a gain adjusting circuit, a vario-repeater for controlling thegain on a signal circuit, means comprising a forward-acting gainincreaser circuit for governing said vario-repeater under control ofsyllabic impulses to raise the gain on the signal circuit, and meanscontrolled directly according to the-signal level on the signal circuitbefore said vario-repeater for reducing the rate of change in gainincrease when the signals are above a predetermined level.

3. In a gain adjusting circuit, a vario-repeater for controlling thegain on a signal circuit, means comprising a forward-acting gainincreaser circuit for governing said vario-repeater under control ofsyllabic impulses to raise the gain on the signal circuit, means forinsuring control ofthe vario-repeater by the gain increaser circuit onlyon the rising portion of a syllabic impulse, and means controlledaccording to the signal level on the signal circuit before saidvario-repeater for reducing the rate of change in gain increase when thesignals are above a predetermined level.

4. In a gain adjusting circuit, control means connected to atransmission line for governing the energy level ofthe signals on theline, a relay for operating said control means, a gain increaser circuitconnected to said line ahead of the connection of the control meansthereto for governing said relay to raise the level of the signals onsaid line beyond the control means, means on said gain increaser circuitto effect control of the relay by syllabic impulses, and means forinsuring control of the relay only on a rising portion of a syllabicimpulse.

5. In a gain adjusting circuit, a vario-repeater for governing theenergy level of the signals on a signal circuit, means comprising arelay for governing said vario-repeater, a gain increaser circuitconnected to said signal circuit ahead of said vario-repeater forgoverning said relay to raise the gain on the signal circuit, means onsaid gain increaser circuit to effect control of the relay by syllabicimpulses, and means comprising a rectifier element connected in serieswith the coil of said relay to insure operation of the relay only on therising portion of a syllabic impulse.

6. In a gain adjusting circuit, a vario-repeater for governing theenergy level of the signals on -a transmission line, means comprising arelay for governing said vario-repeater, a gain increaser circuitconnected to said line ahead of said variorepeater for governing saidrelay to raise the gain on the transmission line, means on said gainincreaser circuit to effect control of the relay by syllabic impulses,means for insuring control of said relay only on a rising portion of asyllabic impulse, and means comprising a second relay controlled by thesyllabic impulses on the gain increaser circuit for reducing the rate ofgain increase when the signals on the line before the vario-repeater areabove a predetermined level.

7. In a gain adjusting circuit, a vario-repeater for controlling thegain on a transmission line, means comprising a control Condenser forcontrolling said vario-repeater according to the polarity and strengthof the charge on the condenser, a gain decreaser circuit for impressinga charge on said condenser in a direction to decrease the gain on theline when the signals on the line beyond said vario-repeater are above apredetermined upper limit, a gain increaser circuit connected to theline before said vario-repeater for impressing a charge of oppositepolarity on said condenser under control of syllabic impulses to raisethe gainon the line, means for insuring charging of the condenser by thegain increaser circuit only on the rising portion of a syllabic impulse,and forward-acting means for reducing the rate of charge of thecondenser by the gain increaser circuit when the signals are above apredetermined level.

8. In a gain adjusting circuit, a vario-repeater for controlling thegain on a transmission l'me, means for controlling said vario-repeaterto lower the gain when the energy level of the signals on the linebeyond said vario-repeater go above a predetermined upper limit, meanscomprising a gain increaser circuit connected to the line before saidvario-repeater for governing said vario-repeater under control ofsyllabic impulses to raise the gain on the transmission line, means forinsuring control of the vario-repeater by the gain increaser circuitonly on the rising portion of a syllabic impulse, and forward-actingmeans for reducing the rate of change of the gain increase by the gainincreaser circuit when the signals are above a predetermined level.

9. In a gain adjusting circuit, a vario-repeater for controlling thegain on a transmission line, a filter connected to the transmission lineat a point before said vario-repeater and tuned to a frequency Withinthe signal frequency range, an amplier connected to the output of saidfilter, a peak chopper connected to the output of said amplier, arectifier for rectifying the output of said amplifier under control ofsaid peak chopper, a

second amplifier' having negative bias supplied thereto by saidrectifier and having an output varying oppositely to signal variationson the line, the output circuit from said second amplifier being tunedto syllabic frequency, a polarized relay connected by a transformer tothe output circuit of said second amplifier, a rectifying elementconnected in series circuit between said transformer and the polar relayto operate said relay on the rising portion of a syllabic impulse, arectifying element connected across the coil of said relay to preventoperation of the relay on the de-

